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I just started to learn how to code with "Intro to App Development With Swift" from Apple on iBooks and whenever I try to edit one of the given practice files, a window pops up saying "The document “02_Naming.playground” could not be saved. The file has been changed by another application. Click Save Anyway to keep your changes and save the changes made by the other application as a version, or click Revert to keep the changes from the other application and save your changes as a version. You can also click Save As to save your changes to a different file." This error pops up every time I type a keystroke in the file. How do I fix this?